EP1610120B1 - Potentiometric measuring probe with external coating as additional electrode - Google Patents

Abstract

The additional electrode (12) is formed by an externally-coated, electrically-conductive layer (14) on the sensor casing (2). The contact terminal (K3) is soldered to the coating. The coating surrounds the entire casing which is transparent, and made of glass. The coating is formed by vapor phase deposition. It is platinum, attached by an intermediate bonding layer comprising titanium, chromium, molybdenum, tantalum or tungsten, optionally in combination with gold or palladium.

Description

Technical area

The invention relates to a measuring probe according to the preamble of claim 1 and to an apparatus for carrying out potentiometric measurements according to the preamble of claim 14.

State of the art

A generic measuring probe is in the US 2003/0132755 A1 especially in the local area Fig. 1 and the appreciation of the prior art. The provision of potentiometric probes with an external, pin-shaped additional electrode (abbreviated to as "solution ground"), among other things, the sensor diagnostics, for example, for measuring and monitoring the resistance of a diaphragm or a glass membrane. In addition, an additional electrode can be used to ground a measurement or process solution in which the probe is immersed, or to set to a defined potential.

A disadvantage of the known probes is that the attached to the lower housing part, pin-shaped additional electrode requires a complicated attachment process in the course of probe production, such as a melting of the additional electrode in the housing. In addition, known from the prior art, the housing projecting additional electrode is a mechanically exposed component of the probe, which brings with it an additional space requirement and is also prone to damage.

In the US 2 563 062 describes a pH electrode, on the outside of which a conductive coating is applied, which extends below the level of the medium to be examined in the immersed state of the probe. This should avoid electrostatic effects on the measuring electronics.

The US 5,316,649 discloses a high frequency reference electrode for electrochemical measurements in which a platinum layer is applied around the shaft and above it a Teflon cover, the latter leaving ring-shaped areas of the platinum coating exposed at opposite ends of the shaft. The coating is used to measure the redox potential and the high-frequency shielding.

Presentation of the invention

The object of the invention is to specify an improved apparatus and an improved method for carrying out potentiometric measurements.
These objects are achieved by the device defined in claim 1 and the method defined in claim 14.

The device according to the invention for carrying out potentiometric measurements has a measuring probe with a housing formed from electrically insulating material which contains at least one cavity with a half-cell element located therein. Furthermore, the measuring probe has at least one additional electrode, which is attached to an immersion region of the housing intended for immersion in a measuring or process solution and is connected to a contact point arranged outside the immersion region. The fact that the additional electrode is formed by an externally applied, electrically conductive coating of the housing, resulting in a compact design of the probe without protruding or otherwise exposed components. The application of the additional electrode is also possible by well-controlled, mature coating process, which also allows the production of large quantities and thus ultimately also results in a cost advantage.

Furthermore, the device according to the invention for carrying out potentiometric measurements has a measuring transducer which can be connected to each half-cell element of the measuring probe and to the electrical contact point of each additional electrode on. The transmitter has means for determining an electrical resistance prevailing between an additional electrode and another part of the device. These include, in particular, diaphragm and membrane resistances as parameters for sensor diagnostics.

Advantageous embodiments of the invention are defined in the dependent claims.

The connection between the coating and the associated electrical contact point can be realized in various ways, for example in the form of a clamping connection. Advantageously, however, according to claim 2, the contact point is soldered to the coating, which is a simple, inexpensive and reliable way of contacting.

In principle, various options are possible for the geometric shape of the coating. In particular, the coating may have the form of a strip extending in the longitudinal direction of the housing or be designed to extend in an annular manner around the housing. According to claim 3, the coating substantially surrounds the entire housing and thereby forms an effective electrical shielding of the measuring probe. It goes without saying that individual housing sites of the measuring probe must remain uncoated for functional reasons. Namely, the diaphragm of a reference electrode and the glass membrane of a pH electrode are to be kept clear of the coating. It is possible to design the coating as a grid-shaped pattern which, despite the existing recesses, allows effective electrical shielding in the manner of a Faraday cage. The defined in claim 3 embodiment is mainly in industrial applications with electromagnetic interference advantageous because the electrical shield reduces the occurrence of interference and improves the stability of the measurement signal. In addition, the comparatively large surface of the additional electrode also allows the measurement of high resistances, which is of considerable advantage in sensor diagnostics.

While electrically conductive coatings inevitably have no light transmission from a certain thickness, it is possible to preserve a certain transparency through a comparatively small layer thickness. The corresponding embodiment according to claim 4 therefore provides the significant advantage that the interior of the probe remains visible, which is particularly advantageous in the case of the all-round coating according to claim 3. It is then not necessary to provide a viewing window in the coating. Visual inspection of the interior of the probe is important in some types of probes, such as saturated electrolyte reference electrodes for determining electrode condition.

Although different types of electrical insulators may be considered as housing materials, it is advantageous if the housing according to claim 5 is formed of glass. In particular, well-known coating methods for the application of the additional electrode can be used.

Advantageously, according to claim 6, the coating is applied by means of a gas-phase deposition, in particular the physical vapor deposition (PVD) and the chemical vapor deposition (CVD). belong. Incidentally, the use of such deposition methods is not limited to glass casings. As another option, the coating could be applied using a brush.

For the coating, various materials come into question, which are generally known as electrode materials. It is particularly advantageous if the coating according to claim 7 is formed from platinum. Platinum is good solderable, has excellent chemical, thermal and mechanical resistance and is also suitable for the measurement of redox potentials. For the determination of metal ion concentrations in the measurement solution, however, a coating of the metal in question is required.

In the measuring probe according to claim 8, an adhesive layer is provided between the housing and the coating, whereby an undesirable detachment of the coating can be avoided.

The material of the adhesive layer also depends, inter alia, on that of the coating, and is selected according to claim 9 from titanium, chromium, molybdenum, tantalum or tungsten. Optionally, the adhesive layer additionally contains gold or palladium. Particularly preferred as a bonding agent between glass and platinum is harmless for biotechnological applications titanium.

The type of half-cell element in the probe depends on the intended application. In particular, the half-cell element according to claim 10 can be designed as a measuring electrode, for example as a glass electrode for pH measurements or as an ion-sensitive ISFET sensor. Furthermore, the half-cell element according to claim 11 may be formed as a reference electrode. Furthermore, the measuring probe according to claim 12 can be designed as a single-rod measuring chain and, for example, contain a pH electrode together with a reference electrode.

The development of the measuring probe defined in claim 13 is provided in particular for the determination of the electrical conductivity in a measuring medium. For this purpose, the measuring probe has two additional electrodes which are attached to different regions of the immersion region and which are each connected to an associated contact point and which define a measuring path for the measurement of the electrical resistance or the electrical conductivity.

A method of monitoring a measuring probe according to claim 14 is based on measurements of an electrical resistance between an additional electrode and another part of the device. In this case, the glass membrane resistance can be measured according to claim 15 or according to claim 16, the diaphragm resistance.

Furthermore, according to claim 17, the device can be used to ground a measuring or process solution in which the measuring probe is immersed or to set it to a defined electrical potential. If the additional electrode consists of platinum can be carried out according to claim 18, the measurement of a redox potential in the measuring solution. According to claim 19, the determination of the electrical conductivity in the measurement solution is possible by means of a resistance measurement.

Brief description of the drawings

Fig. 1

eine als Einstabmesskette ausgebildete Messsonde, im Längsschnitt;

Fig. 2

eine mit rasterförmiger Zusatzelektrode ausgestattete Messsonde, in Seitenansicht;

Fig. 3

eine mit zwei Zusatzelektroden ausgestattete Messsonde, in Seitenansicht.

Exemplary embodiments of the invention will be described in greater detail below with reference to the drawings, in which:

Fig. 1

a trained as Einstabmesskette probe, in longitudinal section;

Fig. 2

a probe equipped with grid-shaped additional electrode, in side view;

Fig. 3

a measuring probe equipped with two additional electrodes, in side view.

Ways to carry out the invention

Matching features are given the same reference numerals in the different figures. For reasons of clarity, the thickness of the coatings is sometimes greatly exaggerated.

The in the Fig. 1 shown measuring probe has a made of glass or plastic tubular housing 2, the lower end 4 is immersed in a measuring medium 6. The probe housing 2 accommodates a central chamber 8 and an annular chamber 10 arranged concentrically around the latter. The central chamber 8 contains a first half-cell element configured as a glass electrode, while a second half-cell element configured as a reference electrode is accommodated in the annular chamber 10. The annular chamber 10 is provided on the outside with an acting as an additional electrode 12 electrically conductive coating 14.

The structure of the two half-cell elements is known per se and will be described below only to explain the entire probe.

The central chamber 8 is formed at its projecting lower end as a dome-shaped glass membrane 16, which is made of a so-called pH glass. Furthermore, the central chamber 8 contains an internal buffer solution 18, for example an aqueous acetic acid / acetate buffer solution containing potassium chloride, in which a first discharge element 20, for example a silver wire, is immersed. The latter is via a non-illustrated melting passage 22 at the upper end of the central chamber 8 led to a measuring contact K1.

The terms "top" and "bottom" are to be understood here with reference to a measuring probe immersed in a measuring medium, oriented approximately perpendicular to the medium surface, as in the US Pat FIG. 1 and are to be adjusted accordingly for a different position of the measuring probe.

The annular chamber 10 has a circular disk-shaped diaphragm 24 in the immersion region 4 and is also provided with a refilling opening 26 in the uppermost part. Furthermore, the annular chamber 10 contains a reference electrolyte solution 28, for example a saturated potassium chloride solution into which a silver wire covered with a silver chloride layer and acting as a second diverting element 30 is immersed. The latter is guided via an upper end part 32 of the annular chamber 10 to the outside to a reference contact K2.

In the example shown, the electrically conductive coating 14 substantially covers the entire outer wall of the annular chamber 10. However, the coating 14 leaves the diaphragm 24 and the refill opening 26 free. The uppermost part of the coating 14 is connected to an additional contact K3, this being advantageously soldered to the coating 14.

It goes without saying that further embodiments can be provided instead of the measuring probe shown here. In particular, instead of a single-rod measuring chain, a single half-cell, for example a glass electrode or a reference electrode, can be provided with an electrically conductive coating acting as an additional electrode. Furthermore, numerous modifications of the arrangement shown here are possible. Thus, the glass membrane may be shaped differently, for example as a ball or needle diaphragm, and the diaphragm may be formed as a circumferential ring diaphragm. The reference electrode but can also be configured as a gel electrode with an open liquid feedthrough, ie without a diaphragm.

The in the Fig. 2 illustrated measuring probe includes a pH measuring electrode with a housing 2a, the lower end 4 is immersed in a measuring medium 6. The sensor chamber 10a formed by the housing 2a contains an internal buffer solution 18 into which a silver wire acting as a diverting element 20 is immersed. The latter is guided to the outside via a non-illustrated upper end part of the sensor chamber 10a to a measuring contact K1. Like from the Fig. 2 As can be seen, the probe housing 2a is provided with an auxiliary electrode 12a which is formed by a grid-like coating 14a having a plurality of small recesses 34. The coating 14a leaves a lower part of the immersion area 4 exposed, but otherwise substantially surrounds the entire probe housing 10a. The uppermost part of the coating 14a is soldered to an additional contact K3. The grid-like coating 14a acts like a Faraday cage and thus provides effective shielding, yet allows for visual inspection of the probe interior.

The in the Fig. 3 illustrated measuring probe includes a reference electrode with a housing 2b, the lower end 4 is immersed in a measuring medium 6. The sensor chamber 10b formed by the housing 2b contains a reference electrolyte solution 28 into which a silver wire covered with a silver chloride layer acting as a diverter element 30 is immersed. The latter is guided to the outside via a non-illustrated upper end part of the sensor chamber 10b to a reference contact K2. The sensor chamber 10b is also provided in the immersion region 4 with a circular disk-shaped diaphragm 24 and in the uppermost part with a refilling opening, not shown. The measuring probe has a first auxiliary electrode 12b and a second auxiliary electrode 12c; which by an associated first coating 14 b (in the Fig. 3 hatched) or second coating 14c (in the Fig. 3 shown black) are formed. Both coatings 14b, 14c extend from the immersion region 4 up to the upper end of the probe housing 2b. The first coating 14b is soldered at its upper end to a first additional contact K3; Similarly, the second coating 14c is soldered to a second additional contact K4.

The first coating 14b comprises a first longitudinal strip 34, which extends from the upper housing part to the immersion region 4 and there merges into an upper annular strip 36, the latter having a graduation 38. The second coating 14 c comprises a second longitudinal strip 40, which also extends from the upper housing part to the immersion region 4 and there merges into a lower annular strip 42. Like from the Fig. 3 shows, the two longitudinal strips 36, 40 are substantially parallel to each other. The second longitudinal strip 40 extends through the division 38 of the upper ring strip 36 and thus reaches the lower ring strip 42. Between the two ring strips 36, 42 is an uncoated zone 44, which is interrupted only by the second longitudinal strip 40. From the Fig. 3 Thus, it can be seen that the first auxiliary electrode 12b is formed by the first longitudinal strip 34 and the split upper ring strip 36; the second auxiliary electrode 12c is formed by the second longitudinal strip 40 and the lower ring strip 42.

For the preparation of the coated probes, the procedure is advantageously as follows. The preferably made of lead-free glass probe housing is first cleaned. Subsequently, a thin adhesion layer of titanium, for example with a thickness of approximately 5 to 20 nm, preferably approximately 10 nm, is applied by a sputtering process first. Finally, the actual coating acting as an additional electrode is applied. Good results with regard to electrochemical and mechanical properties can be achieved, for example, with platinum having a layer thickness of approximately 200 nm. A partially translucent coating will achieve a thinner platinum layer of approximately 50 nm without significant sacrifices in other properties.

1. a) Potentialmessung zwischen K1 und K2 (Fig. 1): Messung des pH-Wertes;
2. b) Widerstandsmessung zwischen K1 und K3 (Fig. 1): Messung des Glasmembran-Widerstandes;
3. c) Widerstandsmessung zwischen K2 und K3 bzw. K4 (Fig. 1 bis 3): Messung des Diaphragma-Widerstandes;
4. d) Potentialmessung zwischen K3 und K2 (Fig. 1 bis 3): Messung eines Redoxpotentials in der Messlösung (Zusatzelektrode aus Platin) bzw. Messung einer Metallionenkonzentration in der Messlösung (Zusatzelektrode aus entsprechendem Metall);
5. e) Anlegen eines definierten Potentials an K3 (Fig. 1 bis 3): Erdung bzw. Potentialausgleich der Messlösung;
6. f) Widerstandsmessung zwischen K3 und K4 (Fig. 3): Bestimmung der elektrischen Leitfähigkeit in der Messlösung.

For use in potentiometric measurements, a probe of the type described above is connected to a corresponding transmitter, which is connected in particular to the various contact points of the probe. In connection with the specifically described probes, the following operations can then be performed:

1. a) Potential measurement between K1 and K2 ( Fig. 1 ): Measurement of pH;
2. b) Resistance measurement between K1 and K3 ( Fig. 1 ): Measurement of the glass membrane resistance;
3. c) Resistance measurement between K2 and K3 or K4 ( Fig. 1 to 3 ): Measurement of the diaphragm resistance;
4. d) Potential measurement between K3 and K2 ( Fig. 1 to 3 ): Measurement of a redox potential in the measurement solution (additional electrode made of platinum) or measurement of a metal ion concentration in the measurement solution (additional electrode made of appropriate metal);
5. e) application of a defined potential at K3 ( Fig. 1 to 3 ): Earthing or equipotential bonding of the measuring solution;
6. f) resistance measurement between K3 and K4 ( Fig. 3 ): Determination of the electrical conductivity in the measuring solution.

LIST OF REFERENCE NUMBERS

2, 2a, 2b

Probe housing, housing

4

Immersion region

6

measuring medium

8th

Central Division

10, 10a, 10b

sensor chamber

12, 12a, 12b, 12c

additional electrode

14, 14a, 14b, 14c

coating

16

glass membrane

18

Inside buffer solution

20

diverter

22

Melt duct

24

diaphragm

26

refill

28

Reference electrolyte solution

30

diverter

32

Upper end part

34

first vertical stripes

36

upper ring strip

38

Division of 36

40

second vertical strip

42

lower ringstrip

44

uncoated zone

K1

measuring contact

K2

reference Contact

K3

first additional contact

K4

second additional contact

Claims (19)

1. Device for making potentiometric measurements, comprising a measuring probe with a housing (2; 2a; 2b) that is formed of an electrically insulating material and has at least one hollow space (8, 10; 10a; 10b) containing a half-cell element, wherein the measuring probe has at least one additional electrode (12; 12a; 12b, 12c) arranged on an immersible portion (4) of the housing (2; 2a; 2b) designed to be immersed in a measuring solution (6) and wherein the additional electrode (12; 12a; 12b, 12c) is connected to a contact terminal (K3; K4) that is arranged outside of the immersible housing portion (4), and further comprising a measurement converter that is connectable to each half-cell element of the measuring probe as well as to the respective electrical contact terminal (K3, K4) of each additional electrode (12; 12a; 12b, 12c), wherein the measurement converter includes means for determining an electrical resistance existing between an additional electrode (12; 12a; 12b, 12c) and another part of the device (K1, K2, K3, K4), characterized in that the at least one additional electrode (12; 12a; 12b, 12c) is constituted by an electrically conductive coating (14; 14a; 14b; 14c) applied to the outside of the housing (2; 2a).
2. Device according to claim 1, characterized in that the electrical contact terminal (K3; K4) is soldered to the coating (14; 14a; 14b; 14c).
3. Device according to claim 1 or 2, characterized in that the coating (14; 14a) surrounds substantially the entire housing (2; 2a; 2b).
4. Device according to one of the claims 1 to 3, characterized in that the coating (14; 14a; 14b; 14c) is at least partially transparent.
5. Device according to one of the claims 1 to 4, characterized in that the housing (2; 2a; 2b) is formed of glass.
6. Device according to one of the claims 1 to 5, characterized in that the coating (14; 14a; 14b; 14c) is applied by deposition from the gaseous phase.
7. Device according to one of the claims 1 to 6, characterized in that the coating (14; 14a; 14b; 14c) is formed of platinum.
8. Device according to one of the claims 1 to 7, characterized in that an adhesion-enhancing layer is put between the housing (2; 2a; 2b) and the coating (14; 14a; 14b; 14c).
9. Device according to claim 8, characterized in that the adhesion-enhancing layer is formed of titanium, chromium, molybdenum, tantalum or tungsten, which may be used in combination with gold or palladium.
10. Device according to one of the claims 1 to 9, characterized in that the half-cell element is configured as a measuring electrode (8, 16, 18, 20).
11. Device according to one of the claims 1 to 9, characterized in that the half-cell element is configured as a reference electrode (10, 24, 28, 30).
12. Device according to one of the claims 1 to 9, characterized in that the measuring probe is configured as a single-rod measuring chain (8, 16, 18, 20; 10, 24, 28, 30).
13. Device according to one of the claims 1 to 12, characterized in that the measuring probe comprises two additional electrodes (12b, 12c) which are arranged in different areas of the immersed part (4) and which are connected to respectively associated contact terminals (K3, K4).
14. Method of monitoring a measuring probe for potentiometric measurements, with a measuring probe and with a measurement converter, wherein the measuring probe comprises a housing (2; 2a; 2b) that is formed of an electrically insulating material and has at least one hollow space (8, 10; 10a; 10b) containing a half-cell element, wherein an additional electrode (12; 12a; 12b, 12c) is arranged on an immersible portion (4) of the housing (2; 2a; 2b) designed for immersion in a measuring solution (6), and said additional electrode (12; 12a; 12b, 12c) is connected to a contact terminal (K3; K4) arranged outside of the immersible portion (4), wherein the measurement converter is connected to each half-cell element of the measuring probe as well as to the respective electrical contact terminal (K3, K4) of each additional electrode (12; 12a; 12b, 12c), characterized in that by means of at least one additional electrode (12; 12a; 12b, 12c) which is formed by an electrically conductive coating (14; 14a; 14b; 14c) applied to the outside of the housing (2; 2a), a resistance value is measured between an additional electrode (12; 12a; 12b, 12c) and another part (K1, K2, K3, K4) of the device, said resistance value serving for the diagnosis of sensor properties.
15. Method according to claim 14, characterized in that the measurement converter is connected to the contact terminals (K1, K3) of the measuring probe, while a resistance measurement is performed between the contact terminal (K1) and the contact terminal (K3) of the additional electrode (12; 12a; 12b, 12c) in order to determine the glass membrane resistance.
16. Method according to claim 14, characterized in that the measurement converter is connected to the contact terminals (K2, K3 or K4) of the measuring probe, while a resistance measurement is performed between the contact terminal (K2) and the contact terminal (K3 or K4, respectively) in order to determine the diaphragm resistance.
17. Method according to claim 15 or 16, characterized in that by means of the measurement converter a defined electrical potential is applied to the additional electrode (12; 12a; 12b, 12c) in order to set the measuring solution at ground potential or to equalize the potential within the measuring solution.
18. Method according to one of the claims 15 to 17, characterized in that by means of a measurement of the potential between the contact terminals (K3 and K2) the measurement of a redox potential in the measuring solution is performed wherein the additional electrode (12; 12a; 12b, 12c) consists of platinum, or that a measurement of a metal ion concentration in the measuring solution is performed wherein the additional electrode consists of the corresponding metal.
19. Method according to one of the claims 15 to 18, characterized in that by means of a resistance measurement between the contact terminals (K3 and K4) a determination is made of the electrical conductivity in the measuring solution.

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